1. Field of the Invention (Technical Field)
The present invention relates to removal of arsenic from water, particularly during otherwise conventional lime softening water treatment processes.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Arsenic has long been known as a highly toxic element. Arsenic exists in two soluble and dangerous oxidation states, As3+, which is known as arsenite, and As5+, which is known as arsenate. Both forms are toxic and exist in groundwater, surface water, and wastewater.
Conventionally various techniques have been examined to remove As from water, such as precipitation (e.g., salts of iron, aluminum, or copper) and coagulation and filtration processes. However, these conventional methods are generally unable to successfully remove the As to lower levels due to the affinity and solubility limitation of the resultant products. The procedures are also time-consuming and expensive, and so not cost-effective.
One method for removing arsenic species from an aqueous medium is the use of an alumina sorbent, as discussed in U.S. Pat. No. 5,556,545, to Volchek, et al. However, the method has some inherent limitations, requiring regeneration and conditioning of the sorbent. Therefore, this regeneration process creates a hazardous solution. Furthermore, the regeneration process results in loss of the sorbent, thus increasing the cost of using activated alumina as a method for removing arsenic from an aqueous medium.
Another method to remove arsenic species from an aqueous medium is ion exchange. One of the disadvantages of this process is that the ion-exchangers utilized are mostly synthetic resins and hence are very expensive. See, e.g., Japanese Patent Application Publication No. H06-304573, to Masafumi, et al. Furthermore, few resins are selective in arsenic removal. A variety of anions such as sulfates compete for the ion-exchange sites in the resin. In general, ion-exchange is not a feasible method of removing arsenic from an aqueous medium if the medium contains a high level of dissolved solids or sulfate concentrations.
Another method for removing arsenic species from an aqueous medium is through the use of a membrane process. A membrane process involves passing the aqueous medium through the membrane to filter the selected material. However, membrane processes are costly as a method for removing arsenic species from an aqueous medium.
Another recently disclosed method is the use of zirconium hydroxide as a paste in water filters, as disclosed in U.S. Pat. No. 6,383,395, to Clarke, et al. The media includes a material selected from zirconium hydroxide, titanium hydroxide, hafnium hydroxide and combinations thereof. The media is preferably in powder form while used to treat water. The media needs to be regenerated repeatedly in order to reduce the cost, while it creates hazardous solutions that need to be disposed of at a cost. Because the media used is in the form of a paste, it does not have high hydraulic permeability and requires use at high pressure. This significantly limits the use of the material to small, high-pressure systems.
Lime softening is employed to remove excess calcium and magnesium from drinking water Upon addition of lime (CaO (quicklime) or Ca(OH)2 (hydrated lime)), calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2, precipitates form that are subsequently removed through microfiltration or by conventional settling followed by filtration. See, generally, xe2x80x9cLime Softeningxe2x80x9d, National Drinking Water Clearinghouse Tech Brief Eight (1998). Typically, to precipitate out calcium carbonate the pH must be increased to 10.0; to precipitate magnesium hydroxide requires a pH of 10.5. L. D. Benefield, et al., Chemical Precipitation, Water Quality and Treatment: A Handbook of Community Water Supplies, American Water Works Association, Fifth Edition, Pages 10.1-10.60 (1999). Low carbonate waters require the addition of soda ash xe2x80x94Na2CO3xe2x80x94 as well. Lime softening removes some arsenic from waterxe2x80x94approximately 5-33% at pH 10 where calcite forms. L. S. McNeill, xe2x80x9cUnderstanding Arsenic Removal During Conventional Water Treatmentxe2x80x9d, Master""s thesis, University of Colorado, Boulder (1996). Greater arsenic removal with lime softening typically requires the higher pH of magnesium hydroxide formation as well as the presence of ferric chloride. L. S. McNeill, et al., xe2x80x9cArsenic Removal by Precipitative Softeningxe2x80x9d, in Critical Issues in Water and Wastewater Treatment: Proceedings of the 1994 ASCE National Conference on Environmental Engineering, Greenwich, Conn.: Braun-Brumfield Publishers (1994). The difficulty and cost of raising drinking water pH to 10 5, adding ferric chloride, and readjusting the pH down to near neutral prevents the widespread use of lime softening for arsenic removal.
The present invention improves arsenic removal by lime softening and does so at lower pH. This is accomplished through addition of divalent metal ions other than calcium or magnesium, most preferably zinc and/or copper.
The present invention is of a method for removing dissolved arsenic from an aqueous medium, comprising: adding lime to the aqueous medium; and adding one or more sources of divalent metal ions other than calcium and magnesium to the aqueous medium; whereby dissolved arsenic in the aqueous medium is reduced to a lower level than possible if only the step of adding lime were performed. In the preferred embodiment, adding lime increases pH of the aqueous medium to at most approximately 10, more preferably to at most approximately 9, and most preferably to at most approximately 8.7. Preferably no ferric chloride is added to the aqueous medium and no further alteration of pH occurs after the adding steps. One or more of the sources of divalent metal ions may be sources of copper ions, preferably to provide a concentration in the aqueous medium of greater than or equal to approximately 1.0 mg/L, more preferably between approximately 1.5 and 2.5 mg/L, and most preferably between approximately 1.5 and 2.0 mg/L. One or more of the sources of divalent metal ions may be sources of zinc ions, preferably to provide a concentration in the aqueous medium of greater than or equal to approximately 0.5 mg/L, more preferably between approximately 0.5 and 3.0 mg/L, and most preferably between approximately 0.5 and 1.5 mg/L. Dissolved arsenate in the aqueous medium is reduced to a lower level than possible if only the step of adding lime were performed. The method may be employed in surface water treatment plants, wastewater treatment plants, plants for treating pumped groundwater, groundwater in situ remediation systems, water filters, or water softeners, and like facilities and apparatuses. The sources of divalent metal ions are preferably one or more of copper sulfate (CuSO4, CUSO4.5H2), copper chloride (CuCl2, CuCl2.H2O), copper nitrate (Cu(NO3)2.6H2O), copper acetate, zinc sulfate (ZnSO4, ZnSO4.7H2), zinc chloride (ZnCl2), zinc nitrate (Zn(NO3)2.6H2O), and anhydrous zinc nitrate (Zn(NO3)2).
The present invention is also of a composition of matter for removing dissolved arsenic from an aqueous medium, the composition of matter comprising lime one or more sources of divalent metal ions selected from the group consisting of copper ions and zinc ions. The one or more sources of divalent metal ions are preferably one or more of copper sulfate (CuSO4, CUSO4.5H2), copper chloride (CuCl2, CuCl2.H2O), copper nitrate (Cu(NO3)2.6H2O), copper acetate, zinc sulfate (ZnSO4, ZnSO4.7H2), zinc chloride (ZnCl2), zinc nitrate (Zn(NO3)2.6H2O), and anhydrous zinc nitrate (Zn(NO3)2). The composition is preferably in solid form, in form of a concentrated solution, or a combination.
The invention is further of an aqueous medium produced by addition of the composition of matter described above or by the process described above.
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.